My Mt Vernon ae is 3 yrs old. Every once in a while it kind of acts like it wants to do its own thing. No matter what temperature you set the thermostat or if its on manual or auto it just runs at its own temperature, usually cold. It starts, runs for a couple minutes, long enough for blowers to operate then shuts down. it just doesn't seem to do anything like the thermostat says. any advice would be greatly appreciated.